The present invention relates to an aluminum alloy clad sheet, and an aluminum alloy clad structural member produced by forming the material aluminum alloy clad sheet (hereinafter, aluminum may be referred to as Al). The clad sheet described herein refers to a laminate sheet produced by laminating aluminum alloy layers to one another, and bonding the aluminum alloy layers to one another by rolling or the like.
An aluminum alloy sheet is used as a material for a structural member of a transport machine such as a vehicle body and an airframe to achieve lightweight. In such a structural member, higher alloy content for higher strength tends to contradict formability into a structural member or certain ductility of the structural material.
For example, for 7000-series aluminum alloy or extra super duralumin (Al-5.5% Zn-2.5% Mg alloy) for the structural member, the amount of strength-increasing element such as Zn or Mg is increased as a typical method for increasing strength. This however reduces ductility and thus makes it difficult to form such alloy into a structural member. Furthermore, such high alloy causes deterioration of corrosion resistance or a reduction in strength due to room-temperature aging (age hardening) during storage. This significantly deteriorates formability into a structural member, or significantly reduces certain ductility of the structural material. In addition, this leads to low production efficiency of a sheet in a rolling step or the like.
Such a contradiction between high strength and formability (ductility) is extremely difficult to be resolved only by investigating a composition, a microstructure, or a manufacturing method of a simple aluminum alloy sheet (single sheet) such as the 7000-series aluminum alloy sheet and the extra super duralumin sheet.
As a measure to solve this problem, there has been known an aluminum alloy clad sheet (laminate sheet), in which two to four aluminum alloy layers (sheets) having different compositions or properties are laminated to one another.
A typical example of such an aluminum alloy clad sheet includes an aluminum-alloy brazing sheet for a heat exchanger, which has a three or four-layered structure in such a manner that a sacrificial anode material of 7000-series aluminum alloy and a 4000-series aluminum alloy brazing material are cladded on a 3000-series aluminum alloy core.
In addition, Japanese Unexamined Patent Application Publication No. 2004-285391 suggests an aluminum alloy material for a vehicle fuel tank, which includes a clad material including a core formed of a 5000-series aluminum alloy material for high strength, and a skin material formed of a 7000-series aluminum alloy material for high corrosion resistance.
Japanese Patent No. 5083862 suggests a method of manufacturing a clad sheet, in which differences in melting point between aluminum alloys such as 1000-series, 3000-series, 4000-series, 5000-series, 6000-series, and 7000-series are used to laminate at most four aluminum alloy layers in one by continuous casting with a twin roll.
Japanese Unexamined Patent Application Publication No. 2013-95980 suggests that when a plurality of aluminum alloy layers are laminated to one another, a Cu anti-corrosion layer is provided between such aluminum alloy layers, and Cu in the Cu anti-corrosion layer is diffused into the bonded aluminum alloy layers by high-temperature heat treatment to improve corrosion resistance of the clad sheet.
However, the contradiction between high strength and formability (ductility) must be resolved so that such existing aluminum alloy clad sheets have both the properties in order to use the aluminum alloy clad sheets for the structural members of the transport machines.
Hence, Japanese Unexamined Patent Application Publication No. 2015-108163 (JP-A-2015-108163) suggests a material aluminum alloy clad sheet having such two properties for structural members of vehicles, or an aluminum alloy clad structural member itself produced by forming (press-forming) the clad sheet as a material.
JP-A-2015-108163 aims to achieve both high strength and high press formability or ductility, which cannot be achieved by a single aluminum alloy sheet, by laminating aluminum alloy sheets having different compositions to one another, the aluminum alloy sheets including an Al—Mg alloy sheet, an Al—Zn alloy sheet, and an Al—Cu alloy sheet.
Specifically, as illustrated in FIGS. 3 and 4 as described later, three to seven Al alloy layers, which have specific and different compositions (each containing one or both of Mg: 3 to 10 mass % and Zn: 5 to 30 mass %) such as an Al—Mg alloy layer and an Al—Zn alloy layer, are laminated to have a total thickness of 1 to 5 mm.
Such a laminate sheet is subjected to diffusion heat treatment so as to have an interdiffusion region in which Mg and Zn in the adjacently laminated aluminum alloy layers interdiffuse, and thus have a microstructure in which hardness of a bonding interface between the laminated aluminum alloy layers is higher than hardness of each of the laminated aluminum alloy layers configuring the bonding interface.